Electrical heating element



April 13, 1965 w. G. MATHEsoN ETAL 3,178,665

ELECTRICAL HEATING ELEMENT Filed Aug. 27, 1962 4 sheets-sheet 1' 1N VENTORS ATTORNEY April 13 1965 w G. MATHEsoN ETAI. 3,178,665

ELECTRICAL HEATING ELEMENT 4 Sheets-Sheet 2 Filed Aug. 27. 1962INVENTORS N O S E H T A M G. D R F H. W

JAMES P. CLUNE THEODORE J. PRICENSKI FIG. 3

ATTOR NEY April 13 1965 w. G. MATHEsoN ETAL 3,178,665

ELECTRICAL HEATING ELEMENT 4 Sheets-Sheet 3 Filed Aug. 27, 1962 MIM6WILFRID G. MATHESON JAMES P. CLUNE THEODORE J. PRICENSKI INVENTORSATTORNEY 4 Sheets-Sheet 4 INVENTORS ATTORNEY April 13, 1965 w. G.MATHEsoN ETAL ELECTRICAL HEATING ELEMENT Filed Aug. 2'?, 1962 (KQ 10") AWILFRID G. MATHESON JAMES F. CLUNE THEODORE J, PRICENSKI BY/av FIG.5

3,178,665 ELECTRlCAL HEATING ELEMENT Wilfrid G. Matheson, Marblehead,James P. Clune,

Danvers, and Theodore J. Pricenski, Ipswich, Mass.,

assignors to Sylvania Electric Products Inc., Salem,

Mass., a corporation of Delaware Filed Aug. 27, 1962, Ser. No. 219,40431 Claims. (Cl. SSS-299) This invention relates to heating elements forelectrical furnaces and more particularly to self-supporting heaterelements suitable for use as cathodes and fabricated of a plexus ofrefractory metals such as molybdenum, tantalum,rhenium, columbium orpreferably tungsten.

Electrical heating elements are known to the art and some types ofelements have previously been fabricated of the above-mentionedrefractory metals. Elements using refractorymetals, particularlytungsten however, have lbeen rfabricated yfrom sheet materials ormachined from heavy stock `and the metals so fabricated do not possesscrystal structures having optimum characteristics to resist breakage ormaintain their geometrical form. But refractory metal wire does possessthese characteristics and we have discovered that foraminous plexusesmade rom wire may be used successfully as heating elements in vacuumand/ or inert or reducing atmosphere furnaces of the resistance orelectronic types. This discoverey led to the achievement of ruggedheating elements which can withstand temperatures as high as 2000o C.and often as high as 2500 to 3000 C. Such foraminous plexuses can bemade to have electrical characteristics substantially identical to thoseknown to the prior art, but yet be structurally far superior to theirsolid sheet counterparts. f

A foraminous plexus according to this invention cornprises a series ofintertwisted, elongated, helical convolutions of refractory metal wires.To form the plexus, a number of helically convoluted wires each havingsimilar electrical characteristics are intertwisted together in such away that generally two convolutions are intertwisted in each otherconvolution except in the case of the rst and last convolutions ofseries, where only one convolution will be intertwisted. Thicknesses ofWire may vary depending upon the application 4and when desired, doubleor triple helical convolutions can he used advantageously to improve thestrength of the plexus. For some applications, it may be desirable toinsert straight support wires in the turn abutments to make the plexusmore durable.

The use of a series of helical convolutions allows greater latitude inthe application of the heater element. Primarily, even after heating andhence when each of the individual convolutions are quite brittle, theconvolutions are movable in their adjacent convolutions and thus canwithstand stresses which would ordinarily fracture solid sheets ofsimilar refractory metals. Coupled with the inherent flexibility of theplexus is the increased strength of drawn wire due to the incorporationof long, fiber-like crystals as a result of processing. Thus, eventhough the individual convolutions of wire may become brittle afterheating, the plexus itself is still very flexible and can be movedfairly freely in the furnace Without an inordinate danger of breakage.Further, the problem of thermal shock, usually resulting from heatingtoo rapidly or cooling too quickly is materially reduced due to theconstruction and crystal structure provided by wire.

Accordingly, the primary object of this invention is the fabrication ofheating elements having increased me- United States Patent O chanicalstrength and the ability to withstand fairly large thermal shocks.

Another object of this invention is the substantial elimination ofdistortion in the geometrical shape of heate ing elements afterelevation to high temperatures in furnaces.

Another object of this invention is the extension of the life ofelectrical heating elements fabricated from refractory metals which canbe heated to temperatures as high as about 3000 C.

A feature of this invention is the fabrication of a plexus of refractorymetals from a series of intertwisted helical convolutions of refractorymetal wire.

A further feature of this invention is the fabrication of a plexus in aforaminous form from a series of helical convolutions of refractorymetal wire.

Another feature of this invention is the use of at least three plexusesconnected together in the shape of a generally cylindrical cage so as toform a cathode for high temperature vacuum furnace.

And yet another feature of this invention is the joining together of aseries of intertwisted helical convolutions by a conductor element whichsupports the series of helical convolutions in the furnace and conductscurrent thereto.

An advantage of this invention is that a heating element of a foraminousplexus can be stronger after heating and have a longer life than similarelements fabricated of sheet metal or machined from heavy stock.

Another advantage of this invention is the wider latitude afforded tothe heating element designer in shaping heating elements of refractorymetals so that many different geometrical configurations can be made.

Many other objects, features and advantages of this invention willbecome manifest to those conversant with the art, upon making referenceto the detailed description which follows and the accompanying sheets ofdrawings in which preferred embodiments of heating elements ofrefractory metal, foraminous plexuses of different coniigurations areshown and described and wherein the principles of the present inventionare incorporated by way of illustrative examples. Of these drawings:

FIGURE 1 is a perspective view of an electrical heating elementparticularly designed for use in a three phase electrical furnace of theresistance or electronic types.

FlGURE 2 is a top plan view of the heating element shown in FIGURE lwith a portion of the upper surface cut away to illustrate thelaminations forming the supporting structure for the plexuses.Furthermore, this ligure shows the Welds made to secure the laminationstogether, which were not shown in FIGURE 1.

FIGURE 3 is a fragmentary View of a portion of `a plexus which may beused in this invention.

FIGURE 4 is a perspective view of another modification of the electricalheater element which may be fabricated according to our invention.

FIGURE 5 is a top plan view of the heater element shown in FIGURE 4 withpart of the Welds which hold the heating element together shown.

Referring now to FIGURE 1 of the drawing, the heating element which iscontemplated by this invention can comprise three foraminous plexuses 1,3 and 5 each of which is formed of a series of intertwisted helicalconvolutions of refractory metal wires (shown in detail in FIGURE 3). Aswill be clearly seen in FIGURE 2, the plexuses can be assembled insubstantially cylindrical form and can be connected together at one endby an outer conductor ring 15 and inner conductor ring (not envasesshown) each of which can be prepared of tungsten when tungsten plexusesare used. Preferably, the attachment of the plexuses to the conductorrings is made by a heliarc welding of the rings together on theirlowermost extremity, however other known techniques for joining andsecuring refractory metal parts may also be used. When welding on atungsten plexus, care should be taken to weld only the lower portions ofthe conductor rings since welding directly on exposed portions of theplexus will tend to disrupt the crystal structure of the tungsten wireand make it fairly susceptible to breakage at the point where thewelding heat is applied. Through a tight tit between conductor rings andwelding the lower portion thereof, the free ends of the helicalconvolutions can be rigidly held together. Usually, a bridge 17 ofintertwisted helical convolutions of refractory metal is disposed in thespace between the inner and outer conductor rings and between adjacentplexuses to realize an effective electrical contact and add stability tothe structure. When formed of helical convolutions, the bridgepreferably is intertwisted into the outer convolutions of the adjacentplexus.

At the other end of the heating element are supporting electricalconducting arms, each of which is disposed upon an individual plexus andis electrically insulated from adjacent plexuses and conducting arms.Preferably, the radius of curvature of a portion of each of the arms isequivalent to the radius of curvature of the lower conductor ring 15 sothat arcuate shapes may be easily formed. Since the heating elementillustrated is particularly adapted to be used in a three-phaseelectrical circuit, we prefer to use three outwardly and radiallyextending conducting arms 19, 21 and 23 disposed upon the plexuses andadapted to be tted into water cooled holders (not shown) which areconnected to a power supply. These holders also serve to retainconducting arms 19, 21 and 23 and support the downwardly depend ingheating element in the furnace. The arms 19, 21 and 23 comprise outerconductive segments 7, 9 and 11 and inner conductive segments 39, 40 and42 formed of shaped refractory metal stock and are preferably tiedtogether by closely wrapped refractory metal wire disposed around theoutside. The windings 25, 27 and 29 tie the outward extensions of theouter segments together and windings 31, 33 and 35 tie the innersegments together as will be explained further in the description ofFIGURE 2. It is apparent however, that other means may be used to retainthe inner and outer segments together such as clips or possibly welding,however the windings shown are preferable since they afford radiant heatdissipation at the outward extremities and help to prevent overheatingof water cooled holders (not shown) which support the heating element inthe furnace.

As we have stated, the heating element according to our invention isparticularly designed for resistance or electronic heating in athree-phase type of vacuum furnace. However, when other than three-phaseheating is desired, the shape of the heating element and the number ofplexuses can be appropriately changed such as for example, by reducingthem to two. Many complex shapes can be fabricated by properlyintertwisting the individual convolutions of refractory metals. Forexample, shapes such as rectangular, corrugated or square can easily bemade and it is even possible in cases where the heater element is not tobe subjected to extremely high temperatures to eliminate the lowerconducting rings and form the plexuses into a conical shape with theapex serving as the conducting ring. In such cases, the plexuses shouldbe insulated from each other throughout their length except at the apex,as is done with the cylindrical cage shown. Another modification is tofabricate the plexuses in a spheroidal shape by making loose windingsand then appropriately bending the convolutions in a jig.

Although it is preferable in most cases to dispose the helicalconvolutions so that they vertically extend from the conducting arms 19,21 and 23 to the conducting rings 15, some requirements of furnacedesign may dictate the use of horizontally extending convolutions, inwhich cases the plexuses will have to be shaped in a jig to conform tothe arcuate or other shapes desired. Of course, the vertical positioningis usually better since the plexuses can be more readily shaped and eachconvolution will be freer to turn in the adjacent convolutions. Whenrigidity of the distal ends of the plexus is a desirable feature, theends may be turned back so as to form cylinders with the last turn ofthe plexus intertwisted into the body of the plexus.

We prefer to fabricate each plexus of a multiphase furnace in a way sothat each has substantially identical electric characteristics. Priorart heating elements for multiphase type furnaces were difficult tofabricate with grids of identical electrical characteristics becauseeach grid has to be individually rolled or machined. Slight variationsin thickness from grid to grid for example, could lead to differentelectrical characteristics in each, since the resistivity of the griddepends upon among other things, cross section and length. Withconvolutions of wire however, this problem is materially reduced becausewire may be drawn to very close tolerances by using techniques nowcommon in the art. And then each of the plexuses for a given multiphaseelement can be formed by intertwisting the same number of helical wireconvolutions for each. Calculation of the precise number ofconvolutions, their length and cross section can be performed routinely.

The individual wire helixes may be formed of any 0f the usual refractorymetals such as molybdenum, columbium, tantalum, rhenium or preferablytungsten. Additionally, alloys of such materials having requisitemelting points also have applicability in some cases. The wire diameterordinarily should be about 0.010 to 0.125 inch since such wire sizesoffer optimum characteristics in a furnace. Below about 0.010 inch, aheating element fabricated of these metals will volatilize too readilywhen heated due to the enlarged surface area and hence, life of theelement will be drastically reduced. Above 0.125 inch the wire will bedifficult to work and coil. The thickness of a plexus will varydepending upon the internal diameter of the helix together with thediameter of the wire. It is desirable for most applications of theplexus to form the helical convolutions on mandrels having diameters ofabout 0.025 to 0.500 inch. And while the upper limit may be increased tosuit individual furnace design requirements, it is generally notfeasible to go below the lower limits stated because the wire which willhave to be used will be too tine to make an efficient heater having areasonable life. Pitch of the individual convolutions can Vary fromslightly above 200% (that is the spacing between the turns equalingslightly more diameter of the wire) to a l000% or even greater. lt isapparent however that at the upper limit the wires must have suflicientpitch to allow for intertwisting of several of the convolutionstogether. Preferably for most applications, we use a pitch of about 300%so that a tight foraminous plexus may be formed.

Referring now to FIGURE 2, the plan view of the upper end of ourelectrical heating element is shown. The welding flux has been partiallycut away to reveal the laminations which support the plexus and conductcurrent from a power supply.

Particularly noting plexus 5 and the associated conducting arm 23,support band 37 is disposed on the inside of the cage at the uppermostextremities of the plexus. Each band of the element is preferably madeof the same material as the plexus to which it is attached therebyreducing the possibility of breakage of the element due to the use ofmaterials having dilferent coeicients of expansion. In the heatingelement depicted, each band is generally arcuately shaped and has athickness of about 0.25 to 0.7 5 inch. However the dimensions can bevaried to suit the individual needs of the Kfurnace in which the elementwill be installed together with the size and weight of the plexuses.

Disposed immediately upon the `support band 37 is the plexus 5. SinceVthe individual coils are vertically aligned, the plexus 5 can be shapedeasily to be compatible with the support band 37.

One the outside of each plexus we pre-fer to place a pair of innersegments 39 which are substantial mirror images of each other. Each ofthe inner segments 39 are of a size sutiicient to extend along about onehalf of the perimeter of the plexus and thence radially outward from themiddle to abut against each other. We prefer to have the inner segments39 extend outwardly at abou-t right .angles to the plexuses so that aminimum angle is made for each `segment thus minimizing localizedoverheating and possible ruptu-ring of the segments.

In order to facilitate a further reduction in heat transmission from theheating element to the water cooled holders (not shown) which supportthe depending heating element in the furnace, we prefer to use atungsten wire wrapping 33 on the inner segments near the beginning ofthe outward extension. This wrapping 33 is quite advantageous becausethe necessity for welding elements together immediately near the bendcan be eliminated. Because welding often changes the crystal structureof a refractory metal, such elimination tends to prevent the formationo-f crystals in the refractory metal which cannot withstand extremes inheat too well. The length of the outward extension of the inner `segmentis a matter of choice and design depending upon the size of the furnace.Similarly as with the Vwrapping of wire around the inner segment 39, weprefer to form a wire wrapping 27 around the outer segment 11 to insurethat it stays together and to increase heat dissipation. l

Generally we abut the outer surfaces of the inner segments 39 againstthe inner surfaces of the outer segments which also are mirror images ofeach other. Each of the outer ysegments extend along about one quarterof the perimeter of the plexuses measured from the distal edge. Theouter segments 11 then branch oif from the inner segment 39 .and extendoutwardly to abut against the outer surfaces of the outward extensionsof the inner segments 39. In this manner, current is effectivelyycarried to each and every portion of the plexus 5 since it passesdirectly from the power supply through the outer segments 11 to theouter portions of the plexus 5 and likewise, a generally equivalentamount of current will pass from therpower supply directly through theinner segment 39 to the inner half of the plexus.

A space suiciently wide to provide electrical insulation of the plexusesfrom each other is left between each plexus of the cage. The outerconducting ring 15 together with the inner ring 16 joins each of theplexuses together at their lower ends and the bridge 17 increases anelectrical connection. As we have previously indicated, it is preferablethat the `bridge 17 be yformed of intertwisted helical convolutionssimilar to those used to form the plexuses. And when using helicalconvolutions, the first and last of the bridge 17 can be easilyintertwisted into the convolutions of the adjoining plexuses. Of course,in some cases it may be desirable to eliminate the bridge 17 or tosubstitute a solid sheet or possibly crimp the inner conductive ring 16against the outer conductive ring 15.

Each of the plexuses 1, 3 and 5 can be supported and connected to thesource of power in a manner similar to that described with reference toplexus 5.

Usually, various elements ywhich make up the conductive supportingassembly for the heating element can be connected together by heli-arcwelding along the top. We have found it best to divide each plexus intoan imaginary four quadrants lfor welding purposes. At the outermostquadrants, both the inner and outer conducting segments are Welded tothe upper edge of the plexus and the support band by welding over thetop of the entire width of the assembly. At the inner quadrants, theinner segments alone are welded to the plexus and the support band. Careis particularly exercised to eliminate welding at the right angle bendo-f the inner conducting segment where the segment extends outwardlyfrom the plexus. In this manner, excessive heat `from heli-arc weldingon the inner segment is prevented together with elimination of thepossibility of modication of the crystal structure which can make theplexus less resistive to shock and vibration.

In FIGURE 3, an enlarged fractional view of a typical assembly ofhelical convolutions of refractory metal which can make up a plexus isshown. For purposes of clarity of presentation, each of the four helicalconvolutions is shown with different shading although each issubstantially the same in size, shape and material. Preferably,configurations for our invention are a series of helical convolutionshaving a wire diameter of about 0.035 inch and an internal :borediameter of about 0.040 inch wrapped upon a mandrel at about 2.16 turnsper inch. The pitch will thus be slightly greater than 300%. Now theparameters of the wire sizes can b e varied over a fairly wide range andstill form workable plexuses. For example, wire of a diameter from 0.010to 0.125 inch can be elfectively wrapped on a mandrel of .025 to 0.50inch and even larger while the pitch can be varied from slightly greaterthan touching to 1000% and even greater. Using tight coils of slightlygreater than 200% will have the advantage however of forming a fairlydense plexus while at 1000% the density can be reduced but ilexibili-tyincreased. Although we show a configuration having two convolutionsintertwisted in each other convolution, except Ifor the iirst and lastof the series, many other variations can be used. For example, thesingle wire shown can -be doubled and possibly tripled and themultistranded wire wrapped into convolutions of the desired pitch on amandrel. Other modifications include the insertion of straight orstranded wire into the turn abutments of .the convolutions to alfordadditional strength. And yet, another method of intertwisting the wireinvolves placing the convolutions of one strand into the interdentalspaces between the convolutions of another strand and then joining theconvolutions together by threading a straight wire through the twoconvolutions, which process can be repeated until a plexus of thedesired size is attained. It is thus quite apparent that many variationscan Ibe made in the method of intertwisting the convolutions and manyplexuses of varying pitches and shapes can be readily fabricated.

The heating element herein described has been specilically designed toprovide Ia high power output at a low voltage which is quiteadvantageous in high vacuum applications. Since high power output ismore economically utilized by a three phase system, the heating elementhas been arranged ina cylindrical form so as to provide three equalresistances with one end of the cylindrically formed plexuses connectedtogether by a conductor ring and with the other end thereof providedwith outwardly extending conductive arms which are formed of conductivesegments. As is manifest, there is one terminal for each phase, andhence, when an A.C. three-phase electrical current is fed to the arms,each member -of the heating element will heat to approximately the sametemperature, thereby insuring a substantially uniform heatingthroughout. The uniformity of heating is achieved due to the -fact thatthe electrical resistance throughout every portion of the plexuses issubstantially uniform. Since the plexuses, the arms and the conductiverings are constructed of relatively heavy material, they afford a lowelectrical resistance to the electric current passing therethrough,thereby enabling the described heat to be generated at relatively lowvoltages. In addition the rugged flexibility afforded, the use offorarninous plexuses enables the element to be selfsupporting and thus,no refractory or other electrical insulation need Ibe used for itssupport in the furnace.

Referring now to FIGURE 4, the heating element in this 'Z modificationdiffers from that presented in FIGURE 1 by the use of a group offorarninous plexuses for each phase of the heating element. Theconstruction is less dense than the previously described congurationhowever cost can be reduced. Additionally, the weight is minimized andsince we use more loosely woven strands, the flexibility can beincreased.

The group of plexuses 2, 18, 20 and 26 from one phase for the heatingelement and each plexus in the entire group and the entire element canbe fabricated and assembled identically. Plexus Z is supported at itslower end by a U-shaped bracket 6, which can be made of 0.025 inchtungsten sheet (when tungsten plexuses are used) 0.50 inch wide and0.875 inch long. The ends of U-shaped bracket 6 are wel-ded toconductive ring 4 and pass around the sides and the front of plexus 2 toinsure its retention in the heating element. It is generally quiteimportant to use a single U-shaped bracket for each plexus instead of acontinuous band of tungsten sheet since the former arrangement allows amore even distribution of current throughout the element.

When tungsten plexuses are used, they may conveniently be fabricated bycontinuously winding 0.035 inch tungsten wire upon a steel mandrel 0.040inch in diameter at a pitch of 2.16 turns per inch. ln general, a numberof feet of the coiling is made, for example 21/2 feet, which is then cutinto a length desired for a plexus such as seven inches. The mandrel isthen removed by techniques conventional in the art such as acid-leachingor other convenient processes and the individual helical convolutionsare then ready -for intertwisting. For the plexus shown in FIGURE 4 weprefer to use seven helical convolutions which can be easilyintertwisted in each other in a manner so that two helical convolutionsare disposed in each other helical convolutions, except in the case ofthe rst and last convolution of the series. Of course otherconfigurations and greater than two helical convolutions can be usedwhen desired, as was described previous- 1y.

At the other end of the plexus 2, a U-shaped bracket 8, which can beidentical to U-shaped bracket 6, is used for the upper support and toattach the plexus 2 to the conductive band 10. Welding each clipconnecting each plexus separately to conductive band herein again helpsto prevent an uneven distribution of current throughout the variousplexuses in the element. When using a three phase element such as shown,it is generally preferable to use arcuate shaped conductive bands andringlike conductors for joining the bottoms of' the plexuses together.Yet variations from these preferred embodiments will readily suggestthemselves depending upon the Shape of the furnace in which the elementis to be used and the heat to which it is to be subjected.

Extending radially from the middle of the conductive band 16 is aconductive arm 12 which is adapted to be supported in water-cooledholders (not shown) that are connected to a power supply. The structureand shape of the arm 12 will be described with greater specicity in thedescription of FIGURE 5.

The spacing of the plexuses around a given band 10 should be regular sothat current is evenly distributed and thus to prevent a localizedtemperature increase on any lone plexus when the current passes through.Unless such even spacing is generally followed, one plexus may have ashorter life then another in a group because of unequal heating `andmetal vaporization.

Referring now to FIGURE 5, the regular distribution of the plexuses isshown around the conductive band i0. Each plexus is joined into a groupby U-shaped brackets which are welded at their ends to arcuate portions14 and 16 of conductive arm 12. Conductive arm 12 is formed of twomirror image segments 14 and 16 which are arcuately shaped at one end tocoincide with the radius of curvature of the conductive band 10 and thenextend radially outward from the middle of the band 10 to abut againsteach other. The lower conductive ring 4 can conveniently be made of twoconductive rings, the inner conductive ring 32 being inside of outerconductive ring 34. The use of two conductive rings, so disposed,increases the stability of the heating element.

In joining the plexuses together, we again prefer to heliarc weld overthe entire upper and lower surfaces of the heating elements and whenhigh temperatures are not to be reached, a weld may be made at the rightangle bend of the inner and outer conducting segments.

lt is apparent that although we prefer to use conductive arms such asshown in FIGURES 4 and 5 for heating elements having phases formed ofgroups of plexuses, the arms described in FIGURES l to 3 also haveapplicability in many cases. And it will be seen from the foregoing thatwe have provided in accordance with the instant invention, an elementfor use in a vacuum furnace which has highly novel features, suggestedflexibility and greatly improved element design.

The instant arrangement enables a high degree of heat to be obtained atlow electrical potential and further results in a highly uniform heatingarrangement. In addition, the heating element may be more efficientlybaiiied which is a highly important feature. Also since the heatingelement is self-supporting, no refractory or other insulation need beemployed for its support in the vacuum chamber. While there is shown anddescribed herein certain specific structure embodying the invention, itwill be manifest to those skilled in the art that various modicationsand rearrangements of parts may be made without departing from thespirit and scope of the underlying inventive concept and that the sameis not limited to the particular forms herein shown and described,except insofar as indicated by the scope of the appended claims.

As our invention we claim:

1. An electrical heating element comprising: an elongated, foraminousplexus of an intertwisted series of helical convolutions of refractorymetal wires, the turns of one of said wires being held by the turns ofan adjacent wire; means for connecting said plexus to a power supply andsupporting said intertwisted helical convolutions, said meanstransversely extending across both the upper and lower ends of saidplexus.

2. The heating element according to claim 1 wherein t-he refractorymetal is tungsten.

3. The heating element according to claim 1 wherein the refractory metalwire has a diameter of about 0.010 to 0.125 inch.

4. An electrical heating element comprising: an elongated, foraminousplexus of an intertwisted series of helical convolutions of refractorymetal wires, wherein said helical convolutions are intertwisted intoother helical convolutions, the turns of one of said wires being held bythe turns of an adjacent wire; means for connecting said plexus to apower supply and supporting said intertwisted helical convolutions, saidmeans transversely extending across the upper and lower ends of saidplexus.

5. The heating element according to claim 4 wherein the refractory metalis tungsten.

6. The heating element according to claim 4 wherein each of saidconvolutions except the first and last of a series are intertwisted intotwo other convolutions.

7. The heating element according to claim 4 wherein the refractory metalwire has a diameter of about 0.010 to 0.125 inch.

8. An electrical heating element comprising: at least two spaced,elongated foraminous plexuses formed of a series of intertwisted,longitudinally extending, helical convolutions of refractory metalwires, the turns of one of -said wires being held by the turns of anadjacent wire; a conductor element disposed at one end of said plexuses,supporting said plexuses and joining them together, said plexusesthereby being insulated from each other throughout their length, exceptat their lower interconnected ends, at least two means for individuallyconnecting each of said plexuses to a power supply and for supportingthe upper ends, said means extending transversely across the other endof each of said plexuses.

9. An electrical heater element comprising: at least three spaced,elongated, foraminous plexus formed of a series of intertwi-sted,longitudinally extending, helical convolutions of refractory metalwires, each of said helical convolutions, except the first and last inthe series, being intertwisted with two other helical convolutions, theturns of one of -said wires ,being held by the turns of an adjacentwire; a conductor element disposed at the lower ends of said plexuses,rigidly aixing the lower ends of the Ihelical convolutions together andjoining said plexuses, said plexuses thereby being insulated from eachother throughout their length, except at their lower interconnectedends; at least three means for individually connecting each of -saidplexuses to a power supply and for rigidly aflxing together the upperends of said helical convolutions, said means extending transverselyacross the other end of each of said plexuses.

10. The heating element according to claim 9 wherein the refractorymetal is tungsten.

11. The heating element according to claim 9 wherein the refractorymetal wire has a diameter of about 0.010 to 0.125 inch.

12. An electrical heater element comprising: at least three arcuate,spaced, elongated, foraminous plexuses formed of a series ofintertwisted, longitudinally extending, helical convolutions ofrefractory metal wires, each of said helical convolutions, except thefirst and last in a series, being intertwisted with two other helicalconvolutions, the turns of one of said wires being held by the turns ofan adjacent wire; a generally circular conductor element disposed at thelower end of said plexuses, rigidly aflixing the ends of the helicalconvolutions of each plexus and also joining the plexuses together, saidplexuses thereby being formed into a generally cylindrical cage andbeing insulated from each other throughout their length. except at theirlower interconnected ends; at least three means for individuallyconnecting each of `said plexuses to a power supply and for rigidlyafiixing together the upper ends of said helical convolutions, saidmeans extending transversely across the upper end of each of saidplexuses.

13. An electrical heater element comprising: at least three groups offoraminous plexuses, each group being formed of at least one elongated4series of intertwisted helical convolutions of refractory metal wires,each of said helical convolutions, except the first and last in aseries, being intertwisted with two other helical convolutions, theturns of one of said wires being held by the turns of an adjacent wire;a generally circular conductor disposed at the lower end of said groups,rigidly affixing the lower ends of the helical convolutions together andalso joining each of the series of convolutions in a group together,said groups thereby being formed into a generally cylindrical cage andbeing insulated from each other throughout their length except at theirlower ends; at least three means for individually connecting leach ofsaid groups of plexuses to a power supply and for rigidly aflixingtogether the upper ends of each of the series of helical convolutions ina group.

14. An electrical element comprising: an elongated foraminous plexusformed of an intertwisted series of longitudinally extending, helicalconvolutions of refractory metal wires; means for connecting one end ofsaid plexus to a power supply and rigidly aflixing said convolutionstogether, said means comprising a pair of inner segments extendingtransversely along the periphery of said plexus and thence outwardly atthe middle at substantially right angles to said plexus to abut againsteach other and a pair of outer segments extending transversely along theperiphery of the outer portions `of said plexus and thence outwardlyfrom the middle to abut against the outer surfaces of said innersegments; means for securing the outward extensions of said inner andouter segments together; and means for joining said inner and outersegments to a power supply.

15. The heating element according to claim 14 wherein the refractorymetal is tungsten.

16. The heating element according to claim 14 wherein the refractorymetal wire has a diameter of about 0.010 to 0.125 inch.

17. An electrical heater element comprising: at least two arcuate,spaced, longitudinally elongated foraminous plexuses formed of a seriesof intertwisted, longitudinally extending, helical convolutions ofrefractory metal wires, each of said helical convolutions, except therst and last in a series, being intertwisted with two other helicalconvolutions; a generally circular conductor element disposed at one endof said plexuses, rigidly aiiixing the lower ends of the helicalconvolutions of each plexus together, said plexuses thereby being formedinto a generally cylindrical cage and being electrically insulated fromeach other throughout their length, except at their lower interconnectedends; at least two means individually connecting each of said plexusesto a power supply and for rigidly aliixing together the upper ends ofsaid helical convolutions, each of said means comprising a pair of innersegments extending transversely along the periphery of each of saidplexuses and thence outwardly at the middle at substantially rightangles to each of said plexuses to abut against each other; a pair ofouter segments extending transversely along the periphery of the distalends of each of said plexuses and thence outwardly to abut against theouter surfaces of said inner segments; means for securing the outwardextensions of said inner and outer segments together; and means forsecuring said inner and outer segments to -said plexuses.

1S. The heating element according to claim 17 wherein the refractorymetal is tungsten.

19. The heating element according to claim 17 wherein the refractorymetal wire has a diameter 0f about 0.010 to 0.125 inch.

20. A supporting arm for an electrical heating element comprising: apair of inner segments extending transversely along the periphery ofsaid heating element and thence outwardly at the middle thereof atsubstantially right angles to said heating element to abut against eachother; a pair of outer segments extending transversely along theperiphery of the distal ends of said heating and thence outwardly toabut against the outer surfaces of said inner segments; and means forsecuring the outward extensions of said inner and outer segmentstogether.

21. An electrical heating element comprising: an elongated, foraminousplexus of an intertwisted series of helical convolutions of refractorymetal Wires, the turns of one of said convolutions being held by theturns of an adjacent convolution; means for connecting the ends of saidplexus to a powersupply.

22. The element according to claim 21 wherein there are at least threeconvolutions of refractory metal wires in the series.

23. An electrical heating element comprising: an elongated, foraminousplexus of an intertwisted series of helical convolutions of refractorymetal wires, the turns of one of said wires being held by the turns ofan adjacent wire; means for connecting said wires to a power source.

24. The element according to claim 23 wherein there are at least threeconvolutions of refractory metal wires in the series.

25. The element according to claim 24 wherein the refractory metal istungsten.

26. The heating element according to claim 24 wherein the refractorymetal wire has a diameter of about 0.010 to 0.125 inch.

27. An electrical heating element comprising: an elongated foraminousplexus of an intertwisted series of helical convolutions of refractorymetal wires, the turns of one of said wires being held Iby the turns ofan adjacent 1 1l wire; means for connecting said plexus to a powersupply, said means comprising a pair of conductor elements, each ofwhich is disposed at the distal ends of the wires forming said plexus.

28. The element according to claim 27 wherein the refractory metal istungsten.

29. The element according to claim 27 wherein the refractory metal wireIhas a diameter of about 0.010 to 0.125 inch.

30. The heating element according to claim 21 wherein each of saidconvolutions except the first and last of a series are intertwisted intotwo other convolutions.

31. The heating element according to claim 21 wherein each of saidconvolutions except the first and last of a series are intertwisted intotwo other convolutions.

References Cited by the Examiner UNITED STATES PATENTS Dreusike 5-189 XHowe 174-129 X Spong 174-129 X Colvin 57-144 Melson 338-295 X Heath338-210 X Weitzel 219-211 Hill 219-553 X Richardson 245--5 RICHARD N.WGOD, Primary Examiner.

15 ANTHONY BARTIS, Examiner.

21. AN ELECTRICAL HEATING ELEMENT COMPRISING: AN ELONGATED, FORAMINOUSPLEXUS OF AN INTERTWISTED SERIES OF HELICAL CONVOLUTIONS OF REFRACTORYMETAL WIRES, THE TURNS OF ONE OF SAID CONVOLUTIONS BEING HELD BY THETURNS OF AND ADJACENT CONVOLUTION; MEANS FOR CONNECTING THE ENDS OF SAIDPLEXUS TO A POWER SUPPLY.